Amplitude controlled quartz oscillator with broad voltage and temperature range

ABSTRACT

The invention concerns an oscillator including an input terminal, an output terminal, a resonator, and an oscillator circuit including: first and second power supply terminals, two capacitors connected between the first power supply terminal and the input terminal, and respectively the output terminal of the oscillator; first and second active transistors of complementary type, forming therewith an inverting amplifier, first and second means for respectively polarising the first and the second active transistors, a first current source formed by a transistor of the same type as the second active transistor, between the second power supply terminal and the second active transistor, current control means for the second polarising means, characterized in that in an steady operating conditions, said second polarising means are arranged for providing a polarisation voltage across the gate of the second active transistor corresponding to the transistor gate voltage of the first current source to within one voltage shift.

FIELD OF THE INVENTION

The present invention concerns, generally, an oscillator circuit to beused for making a time base for an electronic watch, a portabletelephone or any other electronic device requiring such a time base. Thepresent invention concerns, more specifically, an amplitude controlledoscillator circuit.

BACKGROUND OF THE INVENTION

There is known from the prior art, in particular from CH Patent No. 580358, an amplitude controlled quartz oscillator, shown in a simplifiedmanner in FIG. 6. Such amplitude-controlled oscillators are used sincethey reduce the supply current as soon as the amplitude of theoscillator reaches a reasonable value. The oscillator core consists ofan inverting amplifier with the quartz crystal as feedback and loadcapacitances C1 and C2 for providing the necessary phase difference. Thetransconductance necessary for the amplification stage, and thereby thecurrent consumption of the oscillator, is for a given frequencyproportional to the series resistance of the crystal, the oscillatoramplitude and a capacitive factor that is a function of the loaddetermined by C1 in series with C2. The oscillator core corresponding tothe gain stage is formed by a single active transistor N1. The load is acurrent source I₀ which is controlled by the amplitude regulator. Thefeedback resistance Rf which is necessary for polarising the gate ofactive transistor N1 has to be very high, so as not to load theresonator circuit. The complementary transistor P1 acts as polarisationcurrent source for active transistor N1, via the current mirror that itforms with transistor P2. This gain stage easily allows amplitude to becontrolled and requires low supply voltage. However, in order to ensureoptimum selected transconductance, the supply current has to be twotimes higher than for a solution consisting of two active transistorsdisclosed within CH Patent No. 623 459 as shown in FIG. 7.

In the example of the prior art shown in FIG. 7, the gain stage isformed by two active transistors, a transistor N1 in series with acomplementary transistor P1. The transconductances are added in thiscase. Insofar as the same continuous current passes through bothtransistors, half of the current is necessary in order to obtain thesame transconductance with a current source load. Nonetheless, thissolution requires a considerably higher supply voltage and the use of ahigh capacitance C in parallel with transistor P2, which powers the twoactive transistors, which occupies a large surface area on the circuit,and increases the cost thereof while greatly degrading the power supplyrejection rate.

SUMMARY OF THE INVENTION

It is one of the main objects of the present invention to overcome theaforementioned drawbacks by providing an oscillator with a hightransconductance level without requiring the introduction of anyelements that degrade the power supply rejection rate.

Thus, the present invention concerns an oscillator including an inputterminal, an output terminal, a resonator, and an oscillator circuitincluding:

first and second power supply terminals,

two capacitors connected between the first power supply terminal and theinput terminal, and respectively the output terminal of the oscillator,

a first active transistor connected between the first power supplyterminal and the output terminal,

first means for polarising the first active transistor,

a second active transistor, of complementary type to the first activetransistor and whose current path is series connected with the firstactive transistor,

second means for polarising the second active transistor,

a first current source formed by a transistor of the same type as thesecond active transistor, between the second power supply terminal andthe second active transistor,

current control means for the second polarising means,

characterized in that in an steady operating conditions, said secondpolarising means are arranged for providing a polarisation voltage atthe gate of the second active transistor similar or corresponding to thetransistor gate voltage of the first current source, to within onevoltage shift.

This oscillator allows a high transconductance to be obtained due to theuse of two complementary transistors polarised in active operatingconditions, while not degrading the power supply rejection rate owing tothe use of integrated polarising means for the second active transistor,which further ensures a more reduced necessary surface.

Insofar as this type of oscillator is arranged in numerous portabledevices able to be powered in various ways and able to undergo all typesof constraints, in particular temperature constraints, it is alsoimportant to ensure a broad voltage and temperature operating range forthis type of oscillator. For this purpose, an independent and voltagestable current source is further provided, comprising means ofprotection from overvoltage due to a supply voltage that is too high.

As the overall object of the oscillator is to provide at output a clocksignal at a given frequency, an amplifier for alternating signals isadded for this purpose at the input of the circuit forming the core ofthe oscillator in order to be rid of distortion problems observed at theoutput. Advantageously, an inverting amplifier is provided at theoscillator input terminal including an inverter formed of twotransistors of complementary type, controlled by means of two integratedcapacitive input inverting amplifiers each powered by the output of theother capacitive input inverting amplifier and a power supply terminal,respectively the other power supply terminal to prevent a current peakdemand when the output is switched.

In order to widen the operating temperature range of the oscillator, atemperature detector is advantageously provided, associated with acapacitive divider for adapting the transfer function of the amplituderegulator.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will appear moreclearly upon reading the following detailed description of embodimentsof the invention given solely by way of non-limiting example andillustrated by the annexed drawings, in which:

FIG. 1 is a diagram of the oscillator core according to a preferredembodiment of the invention;

FIG. 2 is an overall diagram of the oscillator according to theinvention;

FIG. 3 is a diagram of the inverting amplifier stage at the oscillatoroutput;

FIG. 4 is a diagram of the voltage and temperature stable currentsource;

FIG. 5 a shows a first example embodiment of the capacitive dividercircuit placed at the input of the amplitude regulator;

FIG. 5 b shows a second example embodiment of the capacitive dividercircuit placed at the input of the amplitude regulator;

FIG. 5 c shows the evolution of the oscillator current curve as afunction of the temperature dependent oscillator voltage;

FIGS. 6 and 7, already described, show diagrams of oscillator coresaccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention, which will now be explained, is given purely byway of non-limiting illustration with reference to FIGS. 1 to 5.

FIG. 1 shows the oscillator core according to a preferred embodiment ofthe present invention. The oscillator includes an input terminal(osc_in), an output terminal (osc_out), a resonator 1, and an oscillatorcircuit, also called the oscillator core. The oscillator circuit ispowered via two power supply terminals Vss and Vdd. Two capacitors C1and C2 are connected between one of the power supply terminals Vss andthe input terminal osc_in, respectively the output terminal osc_out, ofthe oscillator. These indispensable capacitors could be elementsconnected as indicated, or could be formed by stray capacitances, inparticular that of active transistor N1 and connections, connectedbetween power supply terminal Vss and the output terminal osc_out.Polarising means, advantageously formed by a diode connected between thegate of active transistor N1 and the output terminal osc_out could beprovided for polarising active transistor N1. A second active transistorP1, of complementary type to active transistor N1, is arranged with aseries connected current path to active transistor N1. A polarisingcurrent source, formed by transistor P2, of the same conduction type asactive transistor P1, is connected between the power supply terminal Vddand the source of active transistor P1. Capacitors C3 and C4 areconnected between the input terminal osc_in and the gates. Theseoscillator input capacitors allow the oscillation signal to pass acrossthe gates while having continuous independent polarisation, whichreduces the oscillator current consumption without affecting itsperformance.

Ingeniously, the polarising means P3, P4 a and P4 b of active transistorP1 are arranged for providing, during the established or steadyoscillator operating conditions, a polarisation voltage across the gateof active transistor P1 similar or corresponding to the gate voltage oftransistor P2 plus or minus one voltage shift. According to anadvantageous embodiment, these polarising means include threetransistors of the same type as active transistor P1, a first transistorP3, providing the polarisation voltage across the gate of activetransistor P1, polarisation voltage and second and third transistors P4a and P4 b whose current paths are series connected and form a currentmirror with transistor P2. These polarisation means are currentcontrolled by resistor R and transistors N2 and N3 connected betweenpower supply terminal Vss and the drain of transistor P4 b respectivelyof transistor P3. Transistors N2 and N3 are themselves connected bytheir gate to a conventional amplitude regulator 4 the output control ofwhich is a function of the oscillation signal amplitude of theoscillator. It will also be noted that during the start up period of theoscillator polarising means P3, P4 a and P4 b, transistor P2 and activetransistor P1 form a current mirror, which guarantees the start currentfor the oscillator. The polarising means of active transistor P2 thusachieved, allow the use of high resistance for polarisation to beomitted while not degrading the power supply rejection rate.

It will be noted in the example given in conjunction with FIG. 1 thatreference is made to a first active transistor of conductivity N and asecond active transistor of complementary conductivity P. It is clearthat the type of conductivity of these two transistors could beinverted, which involves inversion of the entire oscillator circuit,which would cause no difficulty to those skilled in the art and detailsof which will not be given here.

FIG. 2 shows an overall diagram of the oscillator so as to extend thevoltage and temperature operating range of the latter. It showsresonator 1 and the indispensable capacitors C1 and C1 of theoscillator, oscillator core 3, shown in detail in FIG. 1, amplituderegulator 4 for controlling the oscillator current losc as a function ofthe amplitude at the input of oscillator core 3. The diagram of thisamplitude regulator is conventional, one example is provided in anarticle that appeared in the “IEEE Journal of Solid-States Circuits,vol. SC 12, No 3”, dated June 1977 and entitled “CMOS Analog IntegratedCircuits Based on Weak Inversion Operation” in FIG. 10 of that document.The diagram provided uses one type of conductivity, it is nonethelessevident that an equivalent diagram using the complementary type ofconductivity could also be used depending upon the requirements ofoverall circuit.

The purpose of this overall oscillator circuit is to provide at outputVout a clock signal at a given frequency, for example 32 KHz. Thus, anamplifier for alternating signals 5 is added, placed at the inputterminal osc_in of the oscillator core in order to be rid of distortionproblems observed at output osc_out. Details of this amplifier foralternating signals 5 will be given in conjunction with FIG. 3.

It is one object of the present invention to be able to extend thevoltage and temperature operating range of the oscillator. One recurringproblem with the MOS technology transistors used to make this type ofcircuit arises from the maximum gate and drain voltages tolerated insaturated operation conditions, which are limited compared to thetechnology used, and are thus for example of the order of 3.6 Volts.However, in numerous applications, it is useful to be able to use thisoscillator circuit with a power supply delivering a much higher supplyvoltage, for example of the order of 5.5 Volts. In order to be able toextend the voltage operating range, for example from 3.6 Volts to 5.5Volts, a reference temperature stable current 6 is used, coupled to apolarisation distribution current, the detail of which will be given inconnection with FIG. 4. This current reference is preferably coupled toa polarising circuit for a continuous voltage 7, which fixes thecontinuous component level at the oscillator input osc_in.

It has become clear that the operating limit of the oscillator in thetemperature range is limited in particular because of amplituderegulator 4. This is why, in order to extend the temperature operatingrange, the transfer function of amplitude regulator 4 is adapted bymeans of a capacitive divider 8, placed at the input of the amplituderegulator and whose capacitance value depends upon the temperaturedetected by means of a temperature sensor 9. An example of capacitivedivider 8 adapting the transfer function (see the box in FIG. 2 a) ofamplitude regulator 4 as a function of the temperature detected is shownin conjunction with FIGS. 5 a to 5 c.

FIG. 3 shows the amplification stage for alternating signals connectedto the oscillator input terminal osc_in and supplying the output signalVout used by the portable electronic device in which the oscillator isintegrated. It will be noted that this stage is placed at the inputterminal osc_in so as not to avoid the distortion appearing at theoutput terminal osc_out.

This alternating signal amplification stage includes an invertingamplifier formed of two complementary transistors 10 and 11, for shapingthe output signal and controlled by means of two integrated capacitiveinput inverting amplifiers 12 and 13, each powered by the output of theother capacitive input inverting amplifier and a power supply terminal,and respectively the other power supply terminal. It will be noted thatthe power supply of transistors 12 and 13 is advantageously connected soas to prevent simultaneous conduction of transistors 10 and 11 duringthe transition of the output signal. The advantage of this control withno overlap of transistors 10 and 11 is to prevent a significant increasein power consumption for broad supply voltages. Indeed, the use of aconventional CMOS inverter where control of the gate of the twotransistors is common, would cause the presence of a transition currentas soon as the supply voltage exceeds the sum of the threshold voltagesof the two transistors and would thus greatly increase with the increasein supply voltage (for example between 3 and 5.5 V), which is evidentlyundesirable. A solution consisting in powering this inverter with avoltage reducer regulator is also undesirable because of thecomplication that this would involve in reducing the supply voltage.

Advantageously, the inputs of inverting amplifiers 12 a, 13 a are alsopolarised by polarisation means 14, respectively 15, thereby not onlyomitting the use of a large resistor but also directly using the supplyvoltage available in the rest of the oscillator circuit without any needto provide a supply voltage reducer regulator circuit. Thesepolarisation means include three transistors of the same type astransistor 10, respectively 11, of the output inverting amplifier, afirst transistor 14 a, respectively 15 a providing the polarisingvoltage for inverting amplifier 12 a, respectively 13 a, and second andthird transistors 14 b and 14 c, respectively 15 b and 15 c whosecurrent paths are series connected, the drain of transistor 14 a,respectively 15 a being connected between transistors 14 b and 14 c,respectively 15 b and 15 c. Transistors 14 a and 14 b are connected tothe power supply terminal Vss via a current source 16 a, respectively 16b. Likewise, transistors 15 a and 15 b are connected to the power supplyterminal Vdd via a current source 17 a, respectively 17 b.

FIG. 4 shows an advantageous embodiment of a voltage and temperaturestable current source for, in particular, extending the voltageoperating range of the oscillator. The current source includes twotransistors 21 and 22, for example of conductivity P, forming a firstcurrent mirror with a gain determined by the features of thetransistors. It further includes two transistors 23 and 24, ofcomplementary conductivity N, operating in slight inverting operationconditions, to which a resistor 30 is added arranged between the twogates of the two transistors 23 and 24.

In order to prevent the current, drain and source terminals oftransistor 23 being subjected to too great a potential difference, inthe case of a supply voltage of the order of 5 Volts between terminalsVdd and Vss, a high voltage transistor 25 is inserted in the branchformed by transistors 21 and 23, mounted in source follower mode toseparate the drain of transistor 23 from supply voltage Vdd less thethreshold voltage of transistor 21. High voltage transistor 25 iscontrolled via a current source I₁ and another high voltage transistor26 mounted in current mirror with high voltage transistor 25. In thismanner, the drain voltage of transistor 23 will be equal to the gatevoltage of high voltage transistor 25 approximately equal to a thresholdvoltage less its own threshold voltage.

In order to prevent the current drain and source terminals of transistor22 being subjected to too great a potential difference, although this isless critical than for transistor 23, because of the potential dropthrough resistor 30, protection is nonetheless provided by using anelongated channel transistor for transistor 23.

Thus, this current source has the advantage of providing a solution witha broad voltage range.

As the polarisation current distribution has this current source asreference, the rest of the oscillator circuit is made by a currentmirror structure, via a transistor 27 forming a current mirror with thetransistor 22 associated with a current mirror formed by transistors 28a and 28 b. As for transistor 23, in order to protect the whole of thecircuit from potential differences that are too great, due to the use ofa higher power supply, transistor(s) 28 b are protected by high voltagetransistor(s) 29 connected in source follower mode and with the gatethereof connected to the same potential as that of high voltagetransistor 26. This also improves control of the polarisation current byensure the same drain voltage for the current mirror transistors.

FIG. 5 a shows a first example diagram of a capacitive divider arrangedbetween the temperature sensor and amplitude regulator 4. Thiscapacitive divider introduces an attenuation factor for adapting theamplitude regulator transfer function. According to this example, thecapacitive divider includes a first capacitor 31 of determinedcapacitance value and a second capacitor 32 with variable capacitancewhose capacitance value depends on the temperature detected. Thus thecapacitance value of variable capacitor 32 is increased when thetemperature drops, so as to reduce the attenuation factor resulting fromthe capacitance ratio C31/C32 and conversely the capacitance value ofcapacitor 32 is decreased when the temperature increases, so as toincrease the attenuation factor. As a result, the transfer function ofamplitude regulator 4 is adapted such that the operating point with theoscillator corresponds to a current losc higher than the criticalcurrent Icrit below which the amplitude decreases sharply.

FIG. 5 b shows a second example diagram of a capacitive divider madefrom discrete components. Capacitor 31 is shown again with a givencapacitance value. The variable capacitance capacitor 32 is replaced inthis example by three capacitors 33, 34 and 35 connected in parallel toeach other such that their capacitances are added together. Switches 36and 37 are provided on the branches containing capacitors 34,respectively 35, so that at least three attenuation factors,corresponding to three temperature ranges, can be fixed, depending uponwhether the switches are open or closed.

Thus, for example, a first attenuation factor FA=C31/(C33+C34) obtainedwhen switch 36 is closed and switch 37 is open, is used is for atemperature range from 0° C. to 70° C. A second attenuation factorFA_BT=C31/(C33+C34+C35) obtained when both switches 36 and 37 areclosed, is used for low temperature less than 0° C. A third attenuationfactor FA_HT=C31/C33 obtained when both switches 36 and 37 are open, isused for high temperatures above 70° C.

FIG. 5 c shows the evolution of the oscillator current curve losc as afunction of the oscillator input voltage Uosc_in as a function of theattenuation factor applied (FA, FA_BT, FA_HT) depending upon thetemperature values detected.

It will be understood that various alterations and/or improvementsand/or combinations evident to those skilled in the art can be made tothe different embodiments of the invention explained above withoutdeparting from the scope of the invention defined by the annexed claims.

1. An oscillator including an input terminal, an output terminal, aresonator, and an oscillator circuit including: first and second powersupply terminals, two capacitors connected between the first powersupply terminal and the input terminal, and respectively the outputterminal of the oscillator, a first active transistor connected betweenthe first power supply terminal and the output terminal, first means forpolarising the first active transistor, a second active transistor, ofcomplementary type to the first active transistor and whose current pathis series connected with the first active transistor and formingtherewith an inverting amplifier, second means for polarising the secondactive transistor, a first current source formed by a transistor of thesame type as the second active transistor, between the second powersupply terminal and the second active transistor, current control meansfor the second polarising means, wherein in steady operating conditions,said second polarising means are arranged for providing a polarisationvoltage across the gate of the second active transistor corresponding tothe transistor gate voltage of the first current source to within onevoltage shift.
 2. The oscillator according to claim 1, wherein saidsecond polarising means include three transistors of the same type asthe second active transistor, a first transistor providing at the gateof the second active transistor, the polarisation voltage and second andthird transistors whose current paths are series connected and form acurrent mirror with the transistor forming the first current source. 3.The oscillator according to claim 1, wherein in the start up period saidsecond polarising means, said first current source and said secondactive transistor form a current mirror.
 4. The oscillator according toclaim 1, wherein said current control means are formed by two controltransistors whose gate is connected to an amplitude regulator and aresistor arranged between the second supply terminal and the secondtransistor of said second polarising means.
 5. The oscillator accordingto claim 1, wherein an inverting amplifier circuit is provided at theoscillator input terminal including an inverter formed of twotransistors of complementary types, controlled by means of twointegrated inverting amplifiers with capacitive inputs each powered bythe output of the other capacitive input inverting amplifier and a powersupply terminal, respectively the other power supply terminal to preventa current peak demand during the output switch.
 6. The oscillatoraccording to claim 5, wherein the inverting amplifier circuit includesthird and fourth polarising means for polarising said capacitive inputinverting amplifiers, said third and fourth polarising means arearranged so as to use directly the supply voltage available in the restof the oscillator circuit.
 7. The oscillator according to claim 1,wherein said oscillator includes an independent and voltage stablecurrent source including means of protection against possibleovervoltage due to a high supply voltage.
 8. The oscillator according toclaim 7, wherein said current source includes first and secondtransistors, of a first type of conductivity, forming a current mirrorwith a determined gain, third and fourth transistors of thecomplementary type of conductivity, operating in low inversion mode anda resistor arranged between the gates of said third and fourthtransistors.
 9. The oscillator according to claim 8, wherein theprotection means against overvoltage include a high voltage transistormounted in follower mode for protecting the drain of the thirdtransistor from the high voltage.
 10. The oscillator according to claim9, wherein the protection means against overvoltage are further formedby the second transistor which has an elongated channel.
 11. Theoscillator according to claim 1, wherein said oscillator furtherincludes a temperature sensor associated with a capacitive divider foradapting the transfer function of an amplitude regulator in order toextend the temperature operating range.
 12. The oscillator according toclaim 11, wherein the capacitive divider has at least three adaptationlevels.